1. Field of the Invention
The present invention relates to a CMOS precision current source and, more particularly, to a temperature insensitive CMOS precision current source which varies only with sheet resistance and is insensitive to other process parameters.
2. Description of the Prior Art
Many parameters of analog circuits, such as slew rate and operational amplifier gain, are a strong function of bias current. Therefore, it is important to bias analog circuits with a precision temperature independent current source. One early attempt to provide such a current source is disclosed in U.S. Pat. No. 3,813,595 issued to A. C. N. Sheng on May 28, 1974. In this arrangement, an MOS transistor which has its gate electrode connected to its drain electrode is utilized as a constant current device by means of a resistive network placed between its source and substrate electrodes. A remaining problem with this circuit, however, is that the output current is still a function of the circuit bias voltage, which will drift with changes in temperature.
Circuits which provide a constant reference voltage are also difficult to design, for many of the same reasons. One prior art arrangement for providing a temperature independent voltage reference is disclosed in U.S. Pat. No. 4,158,804 issued to W. J. Butler et al on June 19, 1979. In the Butler et al arrangement, the reference voltage circuit consists of a series-connected long channel MOS transistor and short channel MOS transistor which produce, at their junction, a temperature independent voltage. A differential circuit containing three MOS transistors is then provided with one of the transistors serving as a current source that carries the current to the other two MOS transistors which are in parallel. The gates of the two parallel MOS transistors are connected between the above-defined junction and the output. Current divides between the two parallel MOS transistors in such a way as to cause a constant output voltage to be produced regardless of supply variations. This circuit, however, remains sensitive to processing variations.
An article entitled "A Highly Stable Current or Voltage Source", by L. E. MacHattie appearing in Journal of Physics E: Scientific Instruments, Vol. 5, 1972, at pp. 1016-7, describes an arrangement which may serve as either a current or voltage source. As described, a chain of FETs is utilized, where a zero temperature coefficient is obtained at a certain current value characteristic of the individual transistor. However, the dynamic operating range of this arrangement is rather limited.